Cold production of heavy oil is a non-thermal process for heavy oil recovery in which sand and oil are produced simultaneously. The oil is widely believed to flow to the wellbore and to be drained from the reservoir through a network of highly permeable channels called wormholes. In this process, large volumes of sand are produced. The drive mechanism is generally thought to be solution gas drive. The enhanced reservoir access due to the produced sand, in combination with the solution gas drive, provides sustainable production rates for long periods (Sawatzky, R. P., Lillico, D. A., London, M. J., Tremblay, B. R. and Coates, R. M., “Tracking Cold Production Footprints”, Paper 2002-086 Presented at The Petroleum Society's Canadian International Petroleum Conference, 2002, Calgary, Alberta, Jun. 11-13, 2002).
Despite the commercial success of cold production, the process is confronted with water breakthrough leading to loss of oil productivity and additional water disposal costs. Wormholes often break into edge and/or bottom water or aquifers, leading to water cuts as high as 99% and resulting in many oil wells being prematurely abandoned. In 1997, more than three billion barrels of water were produced with water to oil ratio of 6:1 in Western Canada (AEUB, “Alberta Field/Pool Production and Injection Monthly,” Alberta Energy and Utility Board, Statistical Series, 97-16A, 1997).
Most of the work on water shut-off and reservoir conformance control treatments using polymer gel systems has been conducted on porous media. Some work has been done on blocking fractures (Seright, R. S. “Gel Placement in Fractured Systems,” SPE Production and Facilities, 241-248, November 1995). Given the larger diameter of the wormholes compared to the porous medium, or even to a typical fracture width, polymer gels, without reinforcing materials, would tend to form a weak gel plug.
Previously, Zhou et. al. developed a clay-gel system for water shut-off in fractured reservoirs (Zhou et. al., “Process for Reducing Permeability in a Subterranean Formation,” U.S. Pat. No. 6,143,699 issued Nov. 7, 2000; Zhou, Z. J., “Clay Gel for Water Shut-off in Heavy Oil Production,” presented at the 1998 CIM Heavy Oil Technical Symposium, Lloydminster, Alberta, Canada, Sep. 16-17, 1998).
In this method, the clay gel swells once in contact with formation water through cation exchange between the potassium cations in the clay gel and the sodium cations in the formation to form a clay gel plug. The use of clay gels, which seemingly provide a clay gel plug of sufficient strength for fractures, is limited by the formation salinity, which must be below about 3 weight %. Field-testing of the method showed that a clay gel plug was able to reduce the water cut to 60% for the first three weeks. However, the water cut increased back to 85% afterwards. Two possible explanations, among others, may be given for the ultimate failure of the treatment: 1) a low yield stress (strength) of the clay gel plug; and/or 2) fingering of the fresh water into the clay gel within the wormholes during the post-flush treatment.
An analysis of a tracer study conducted by Amoco (Squires, A., “Inter-Well Tracer Results and Gel Blocking Program,” paper presented at the Tenth Annual Heavy Oil and Oil Sands Technical Symposium, Mar. 9, 1993) investigating the connectivity of wormhole channels between wells provides an estimate of the diameter of the open channel portion of a wormhole between the subject wells. From the injection rate between two wells in one test (30 m3/day), the distance between the two wells (400 m) and the travel time between the wells at the level of the perforations (one hour), the diameter of the open channel portion was calculated to be 6 cm. In this calculation, the injector and producer wells were assumed to be connected by a single wormhole. When two wormholes from different wells connect, the bottom hole pressure within each well would have a tendency to equalize. This equalization would reduce the tendency of forming additional channel connections between wellbores, so that the assumption of a single connecting wormhole between the subject wells is a reasonable assumption.
Based upon the Amoco tracer study, a gel system for plugging or blocking high permeability channels such as wormholes preferably has enough strength as a gel plug to block an open channel having a diameter of at least about 6 cm. Numerical simulations of the erosion at the surface of the open channel portions of wormholes suggest that the open channel portions of wormholes could be as large as 10 cm. in diameter (Tremblay, B., Wiwchar, B., Huang, H., Bani, E., Cameron, S. and Polikar, M., “Development and Testing of a Sandy Polyacrylamide Gel for Water Shut-off of Large Channels,” Alberta Research Council, AACI Report 0102-2, January 2002).
A gel system for blocking high permeability channels such as wormholes should preferably also satisfy the following criteria: a) the gel should be capable of being placed at an appropriate location in order to perform the blocking function; b) the resulting gel plug should have sufficient strength to withstand formation pressure; and c) the gel system and its use should provide relatively low cost with minimum environmental impact.
The proper placement of the gel is dependent upon proper selection of the gel system and proper preparation of the gel. Preferably, the injection pressures for the gel should not be excessive, the gel should be capable of flowing through casing perforations into the high permeability channels, and the gel should be capable of penetrating as far as possible into the high permeability channels.
The strength of the resulting gel plug is dependent upon the composition of the gel. In order to be effective, the gel plug should have enough strength to withstand the formation water pressures typically encountered downhole, which pressures could reach about 3 to 4 MPa in cold production wells. Reinforced gels have been proposed as gel systems which may provide suitable strength as reinforced gel plugs to withstand downhole pressures.